SubPopulations and InteRmediate Outcome Measures In COPD Study (SPIROMICS): Exacerbation substudy The Exacerbation Substudy contributes to the two primary aims of SPIROMICS the identification of 1) homogeneous subgroups of COPD and 2) intermediate biological and clinical outcome measures for use as clinical trial endpoints. Exacerbations are considered sentinel events in the natural history of COPD. Their frequency and severity have been associated with multiple negative implications including worsening health status, worsening pulmonary function, increasing cost of care, and increased mortality. A better understanding of the nature of these events and their symptomatic manifestations has the potential of leading to an improved understanding of the biological genesis of exacerbations and provide novel therapeutic approaches. Participants who consented to being contacted about participating in additional substudies will be approached during Baseline or clinic Visit 2 to determine interest in the Exacerbation Substudy. Interested participants will then be screened and consented for the substudy. A total of 400 participants will be enrolled in Wave 1 and 2. While each contractor is expected to enroll approximately 67 subjects, the underlining goal is to enroll 400 participants. Negotiations are based on each site recruiting 67 participants. Recruitment adjustments may occur in subsequent periods if it is determined that one site is able to exceed the recruitment goal of 67 while other sites are not. Participants enrolled into the Exacerbation Substudy will fall into one of two substrata: 1) Sub-stratum one (COPD patients at risk for acute exacerbation) History of two or more exacerbations in the last 12 months 2) Sub-stratum two (COPD patients not at risk for acute exacerbation) History of one or no exacerbations in the last 12 months Assessments included in this substudy do not require the full cohort in order to achieve the appropriate statistical power.